In powder coatings area, there are always such demands that intended powder coatings should be excellent in blocking resistance and should give the least amount of fused or aggromerated mass or caking during storage thereof and that such powder coatings should be excellent, when applied by an electrostatic powder coating or a fluidized bed coating and baked, in flowability under molten state so that a smooth surface coating which is free from pinholes or creeping can be obtained therewith. However, since the abovementioned are each conflicting properties, none of the heretofore proposed powder coatings could satisfy the both requirements at the same time. That is, when a low molecular weight resin or a low Tg (glass transition temperature) resin is selectively used as a binder resin, flowability of the melted resin is indeed improved to some extent, but blocking resistance and sometimes film properties are liable to be lowered. Therefore, at the present days, it is inevitable that resin planning and paint planning are forced to make by making a compromise with only moderate improvements in blocking resistance, flowability and coating properties. As an approach for the solution of said problems, has been tried the addition of inorganic particles as colloidal silica, siloxane and the like into powder coatings. However, there is a limit in the amount of such particles to be added since an excessively larger amount of such particles will cause undersired loss in gloss or water resistance of the formed coating. Thus, the amount of such particles are only limited to at most about 0.2% by weight, which is totally useless for the intended improvement in blocking resistance of powder coatings.
The inventors have previously succeeded in establishing an effective means for improving both blocking resistance of powder coating and weather resistance , hardness and other desired properties of the formed coating wherein a comparatively larger quantity of crosslinked resin microparticles having an average diameter of 0.01 to 10.mu. are added to base powder coatings, and applied a patent on it ( Japanese Patent Publication (unexamined) Sho 62-292869). According to that invention, the resin microparticles used are crosslinked and hence, such microparticles themselves do not flow at the baking stage. Therefore, a further improved powder coating having excellent flowability and capable of resulting a far better coating with excellent appearance has been desired.
Furthermore, in a household electric appliance area as electric refrigerator, washing machine and the like, a stain resistance of the formed coating is a matter of great concern. Polyester-epoxy type powder coatings are believed to be the most suitable one in respects of corrosion resistance and mechanical properties of the resulted coating and however, even with this type of powder coatings, various problems are pointed in the household electric appliance area, due to its rather poor stain resistance and weather resistance properties. Household electric appliances ( e.g. microwave range, refrigerator and the like) are usually placed at the interior of a house and therefore, they are always exposed to various stains as smoking stain, magic ink stain, foodstuff stain, rouge stain and the like. Stain resistance of coating may be improved by increasing crosslink density of the coating by using a particular hardening system or a strong hardening catalyst or by increasig Tg value of used resin. However, these techniques will accompany the undesired increase in coating viscosity, resulting a marked loss in finishing appearance of the coating. Therefore, a fully satisfiable coating cannot be expected with these techniques from the standview of coating appearance and stain resistance.
In order to obtain an improved stain resistance as well as an improved weather resistance of coating, special powder coatings had been proposed in Japanese Patent Publication (unexamined) Sho 56-14567, wherein a particular hardening agent obtained by the reaction of p-oxybenzoic acid diglycidyl ester ether with a hydantoin compound was compounded with a polyester resin having a specific acid value and softening point. However, in that technique, the employable resins are of specific type and very expensive hardening agents are selectively used and therefore, the disclosed powder coatings can not be welcomed for generic uses. A more effective and less expensive powder coatings should be developed.
Among various properties of formed coating, weather resistance is another important matter of concern in an automobile industry and other technical fields. For this end, various ultraviolet ray absorbers (UV absorber) as benzophenones, benzotriazoles, benzoic acid phenyl and salicylic acid phenyl compounds or various antioxidants as phenols, phosphoric acid compounds, sulfur containing compounds and amines are often compounded in powder coating. However, these additives are comparatively expensive. Furthermore, since the additives must be uniformly compounded with the powder, comparatively larger quantitites of such additives are required for the intended object. Therefore, a more effective and economical solution for the improvement in weather resistance is likewise be desired.
In a separate technical field of household electric appliances, there is an increasing demand for the obtainment of matte surface coating. At that time, the powder coatings used should be, of course, excellent in blocking resistance and the resulted coating should be excelelnt in film properties. To get a matte surface coating, various techniques have been proposed. That is, the first method is to add matting agent in powder coatings, the second one to add a combination of different type of matting agents and the third one is to add a hardening catalyst to powder coatings. Among them, in the first method, a larger quantity of matting agent is required for attaining a sufficient reduction in gloss of the formed coating, which in turn will cause the decrease in coating appearance and desired properties and especially water resistance. Therefore, the quantity of such matting agent is limitted at most about 40% by weight of the total solid, which is rather deficient in having the intended matting effect. Furthermore, in this method, there is an additional problem such that the matting effect is inconsistent depending on the dispersion and kneading conditions. The same is true in the case of the respective second and third methods.There are also additional problems of poor compatibility and hard control of curing reaction in the latter two methods. Thus, fully satisfiable powder coatings have not been offered up to the present days for the matte surface coating.
It is, therefore, a principal object of the invention to provide powder coatings being excellent in blocking resistance and flowability at molten stage, capable of resulting a coating with excellent stain resistance, without the necessity of using any special additives or hardening agent, in an economical way. An additional object of the invention is to provide powder coatings being excellent in blocking resistance, capable of resulting a coating with far improved coating appearance, as well as stain resistance. A further object of the invention is to provide powder coatings which have the abovementioned characteristics and besides that, can result a coating with far improved weather resistance. A further object of the invention is to provide powder coatings being excellent in blocking resistance and capable of resulting a matte surface coating without losing the desired flowability, stain resistance and film properties. The other objects shall be clear from the statements of the specification and accompanying claims.